Adjust alias-uninit-value.rs

- Document and tidy up `alias-uninit-value.rs`
- Move `alias-uninit-value.rs` to `tests/ui/codegen/`

Author: jieyouxu

tests/ui/alias-uninit-value.rs

@@ -0,0 +1,26 @@
+//! Regression test for issue #374, where previously rustc performed conditional jumps or moves that
+//! incorrectly depended on uninitialized values.
+//!
+//! Issue: <https://github.com/rust-lang/rust/issues/374>.
+
+//@ run-pass
+
+#![allow(dead_code)]
+
+enum TyS {
+    Nil,
+}
+
+struct RawT {
+    struct_: TyS,
+    cname: Option<String>,
+    hash: usize,
+}
+
+fn mk_raw_ty(st: TyS, cname: Option<String>) -> RawT {
+    return RawT { struct_: st, cname: cname, hash: 0 };
+}
+
+pub fn main() {
+    mk_raw_ty(TyS::Nil, None::<String>);
+}